Field of the Invention
All metal ingots are cast from molds. These molds rest on bases commonly known as "stools. " The stools are large, normally rectangular, flat slabs of metal usually made of cast iron. The stools are used as support for the mold sides and also to form the bottom portion of the mold. In a "big end down" type of mold, the mold sides generally taper down in diameter from bottom to top. In another type of mold, known as the "big end up" mold, there is a ladle-like receiver for the molten metal, the bottom portion of which is an integral non-removal part of the entire mold.
Various problems commonly occur in use of these molds and particularly with respect to the surface of their base portions. The unprotected metal surface on the base portion quickly erodes and pits in the presence of molten metal which is cascaded upon its surface. Large gouges in the base portions are produced due to the force and high temperature developed by the flowing molten metal which contacts the surface of the stool.
Since many molds are generally 5-10 feet in height, the metal must be poured from a height at least equal to that distance and quite often is poured from even greater heights. A considerable pressure head thereby developed. Thus, the hot molten metal easily gouges gaping depressions in the base members under such force and at a temperature of at least the liquefaction temperature of the molten metal. Moreover, the problem of creation of pits or gouges in the base portions of the molds, caused by the above factors, is aggravated due to the fact that the moten metal, especially near the bottom of the mold, remains in its erosive hot liquid state for a considerable amount of time subsequent to pouring.
The molten metal, after solidification to an ingot has a bottom from conforming to the undesirable eroded surface configuration of the stool or base member of the mold. Thus, a considerable amount of the ingot, when withdrawn from the mold and subsequently processed into slabs or blooms, is lost through a cropping of the irregularly formed end of the slab. This, of course, is highly undesirable, since it results in undue loss of usable metal and an increase in scrap which must be subsequently reprocessed.
Another extremely serious and costly problem arises after the ingot in the mold has solidified to a point where it can be removed from both the mold sides and its base platform member or stool. If the surface of the stool is unprotected or inadequately protected and erosion occurs as described above, the ingot has a greater tendency to remain tightly adherent to the stool. Thus, after the mold sides are removed from around the ingot, which process can normally be efficiently achieved with a minimal film of coating selected from a variety of coating agent, the ingot must be forcibly removed from the stool.
Removal is normally achieved by raising both ingot and adherent stool, and thrusting them against some other larger object whereby the ingot is jarred loose. In many cases the stool and ingot are merely dropped on the floor some suitable height. In such a situation, the stool is often broken into two or more smaller pieces and cannot be subsequently reused in casting other ingots. Again, replacement cost of these stools is high, making this aspect of the overall casting process somewhat disadvantageous. The same problem exists with respect to big end up molds wherein sticking of ingots particularly occurs at their base portion. New molds of this type are especially vulnerable to sticking due to their smooth surface unprotected by any layers of metal oxides or scale. A tight metal-to-metal bond between mold bottoms and ingots then occurs.
Cracking of molds particularly cracking of their base portions due to the above discussed rough handling occasioned by "stickers" between the base portions and ingots is enhanced by thermal shock during ingot formation. Unprotected or inadequately protected bottom surfaces of molds are especially susceptible to such destructive shock.